は and が - Using は before 欲しい instead が

I was doing some exercises and the book that I'm using translated:

I don't want shoes

to

靴は欲しくないです

I answered using が particle instead は, since, as far as I know, I should use が before 欲しい. So why did the book use は this time?

I imagined that the reason is to make a contrast/comparison that は provide. Is that why?

Answer

"I want xxx." vs. "I do not want xxx."

When you want something, you will generally say:

「XXX + が + ほしい」

and when you do not want something, you will generally say:

「XXX + は + ほしくない」

The only times you can and must use 「XXX + が + ほしくない」 is when it is used in the if-clause or relative clause. Thus, it is correct to say:

Relative Clause:「靴{くつ} + が + 欲{ほ}しくない人は靴屋{くつや}へは行かない。」

"A person who does not want shoes would not go to a shoestore."

If-Clause:「靴 + が + 欲しくなければ、なんで靴屋に来たの？」

"If you do not want shoes, why did you come to a shoestore?"

Moving on...

The contrastive 「は」

This is essentially a different topic from what has been discussed above.

It is correct to say:

「靴 + は + 欲しくない。」

when you do not want shoes but want another thing. Thus, you can say:

「靴は欲しくないけど、ジーンズは欲しい。」

That is a pair of contrastive は's. "I don't want shoes, but I want some jeans."

I imagined that the reason is to make a contrast/comparison that は provides. Is that why?

No, as I stated above. We are talking about two different things. One is affirmative vs. negative. The other is contrastive.